(1) Field of the Invention
The present invention relates generally to package sorting conveyors and, more particularly, to a control system for such a conveyor system.
(2) Description of the Related Art
Conveyor systems having a number of individual carrying carts have been commonly used for many years to carry and sort packages or other items, such as mail. For example, U.S. Pat. No. 5,054,601 to Sjogren et al. discloses a package sorting conveyor comprised of a train of tilt tray carriers coupled in tandem to form a continuous loop. Each carrier includes a pivotally mounted tilt tray normally maintained in an upright position. The carriers are moved around the loop by a series of motors spaced around the loop. Branching out from the loop are outfeed chutes or the like for receiving packages from the carriers. When a particular carrier holding a particular package to be sorted reaches a selected outfeed chute, an actuator tilts the tray to dump the package into the outfeed chute. Another example of a typical package sorting conveyor is disclosed in International PCT Application Number PCT/DK90/00047 of Kosan Crisplant A/S.
One significant disadvantage of conventionally designed package sorting conveyors is that conventional conveyor carriers laterally tilt only on a horizontal axis parallel to the direction of conveyor travel. While this accomplishes the objective of dumping the package from the carrier into an outfeed chute or the like, the package is often roughly tumbled or rolled, sometimes damaging the package's contents. One reason for this is that the packages typically are unloaded from the carrier while still traveling forward at the same speed as the conveyor. Thus, packages tend to slam into a forward retaining wall of the outfeed chute before sliding down the chute. Another problem with conventional laterally tilting conveyors is that because the packages are moving forward at full speed when they are unloaded into the outfeed chute, the outfeed chute must be relatively wide so that packages do not miss the chute and fall off the conveyor past the chute. This often unnecessarily increases the overall size of the conveyor system.
U.S. Pat. No. 4,744,454 and an improvement thereto, U.S. Pat. No. 5,086,905, both to Polling, disclose previous attempts to remedy this problem of rough handling by conventional laterally tilting conveyor carriers. Both of these patents to Polling disclose a conveyor element for a package conveyor that includes a tilting carrier tray mounted to be rotatable about two swivel axes. A first swivel shaft extends obliquely downward from the underside of the carrying tray and is in turn connected at an angle to the end of a second swivel shaft extending obliquely upwards from a base support part of the conveyor element. Together, the two swivel shafts form a "V" that points in the direction of conveyor travel. Both of the swivel shafts lie in the vertical plane of symmetry of the conveyor element when the carrier tray is disposed in its upright position.
Because the carrier tray of Polling rotates about two oblique axes, the carrier tray can be tilted not only lateral on a horizontal axis, but is moved through a geometrically complex spatial reorientation during package discharge. This allows for more gentle placement of a package on an outfeed chute than can be accomplished using conventional conveyor trays that laterally tip on only a horizontal axis. The Polling conveyor element more gently handles the packages by imparting some degree of rearward velocity to the packages as they are discharged, which, when added to the forward velocity of the conveyor system, results in the packages' forward velocity during discharge being less than that of the conveyor system itself.
However, the conveyor elements of both of Polling's patents are unduly complicated and intolerant of manufacturing discrepancies. In fact, the second Polling conveyor element (U.S. Pat. No. 5,086,905) was invented in an attempt to simplify the original design disclosed in the first Polling patent (U.S. Pat. No. 4,744,454), which had proved to be too expensive and complicated to manufacture efficiently. As a result of this complexity and cost, the Polling devices have not enjoyed significant commercial acceptance and success.
One solution to this problem is shown in co-pending U.S. application Ser. No. 08/632,012, filed Apr. 15, 1996, now U.S. Pat. No. 5,836,436 issued Nov. 17, 1998, the entire disclosure hereby incorporated by reference.
Another problem which is more recent has arisen from the movement of the industry from chain driven sorters to linear induction motor (LIM) driven sorters. LIM's are extremely quiet and have fewer moving parts which require maintenance. However, such systems are not trouble free and have a tendency to overheat when run at lower speeds. It is now believed that the conventional practice of using an AC inverter to control by varying frequency produces transients which contribute to this heating problem which occurs predominately in linear induction motors. In addition, in such "chain-less" systems it has proven more difficult to accurately determine the speed and position of the trays since there is no physical connection between the trays and the motor.
Thus, there remains a need for a new and improved control system for a tilting conveyor system that substantially eliminates the heating problem when the LIM is run at slower speeds while, at the same time, accurately determines the speed and position of the trays.